I have done the hacks for both Tivo and Replay (Showstopper).
The hacks are straightforward provided the following concepts/
procedures don't sound foreign to you: master/slave drives,
rejumpering hard drives, making a boot floppy, booting into
Linux off a floppy (or, alternatively, running a program
directly under Windows 2000 for the Replay hack), opening a
computer and switching cables/drives around.
If this sounds scary to you, forget doing the hack. You're not
at the appropriate level. If this sounds trivial, you'll find the
hack simple.
The hardest part of either hack is mounting the second drive in the
case. This is not really hard for the Tivo--there's a mounting
platform in the case. All you need to do is figure out how to
secure the drive to the platform (I use plastic ties and some
felt stickies, but you can also buy a grossly overpriced mounting
bracket for $15).
For the Replay/Showstopper, mounting the second drive is more
difficult. That's because there's no second mounting platform
in the unit. You have to think a bit to figure it out. See
links in
www.avsforum.com for more details. It's not hard once
you think about it, but you WILL have to think a bit.
Note that the Replay/Showstopper hack does NOT require you to add
a second drive. You can simply replace the single drive in the unit
with a larger drive, if you prefer, and this avoids the issue of
mounting the second drive altogether. Thus, you can take your 20
hour unit, replace the hard drive with a 80Gb drive, and have
an 80 hour unit without having to mount a second drive.
Kwad